The compound 2,6-diaminopyridine (“DAP”), which is represented by the structural formula shown below:
is a useful starting material for preparing monomers for rigid rod polymers such as those described in WO 94/25506, as well as for the preparation of dyes, metal ligands, medicines and pesticides.
It is known to prepare DAP by means of the Chichibabin amination reaction, in which pyridine is reacted with sodium amide in an organic solvent. This is a complicated reaction, however, requiring relatively severe conditions (e.g. 200° C. at elevated pressure). Additionally, handling the sodium amide and isolating the desired product from this complex mixture are difficult operations to perform on a commercial scale.
The synthesis of 2,6-diaminopyridine and related compounds from glutaronitriles or glutarimidines may be described as proceeding via a dehydrogenative aromatization reaction. A process for the preparation of 2,6-diaminopyridine and related compounds from glutaronitriles and related compounds by contacting an acyclic dinitrile compound with a chemical oxidant and/or a dehydrogenation catalyst in liquid ammonia neat or in a mixture of ammonia and a polar, aprotic solvent, and heating the reaction mixture in a closed vessel, is described in U.S. Provisional Application No. 60/876,577 (filed 21 Dec. 2006), which is by this reference incorporated in its entirety as a part hereof for all purpose. A process for the preparation of 2,6-diaminopyridine and related compounds from glutarimidines and related compounds by contacting a glutarimidine with a chemical oxidant and/or a dehydrogenation catalyst in liquid ammonia neat or in a mixture of ammonia and a polar, aprotic solvent, and heating the reaction mixture in a closed vessel, is described in U.S. Provisional Application No. 60/876,557 (filed 21 Dec. 2006), which is by this reference incorporated in its entirety as a part hereof for all purpose.
British Patent 2,165,844 discloses the gas-phase conversion of glutaronitrile to pyridine in the presence of hydrogen over a palladium on silica catalyst. U.S. Pat. No. 4,876,348 discloses a gas-phase process for making 3-cyanopyridine by dehydrocyclization of 2-methylglutaronitrile over supported catalysts to make a mixture of 3-methylpyridine and 3-methylpiperidine, and reacting such mixture with ammonia over various oxide catalysts. U.S. Pat. No. 5,066,809 discloses a gas-phase process for making 3-methylpyridine by contacting 2-methylglutaronitrile in admixture with hydrogen gas with a supported catalyst.
Despite these existing processes to make pyridines, a need remains for a process for the gas-phase preparation of aminopyridines, and in particular DAP and related compounds.